U.S. Pat. No. 3,764,566 teaches the addition of silicon dioxide to zinc oxide varistors along with barium oxide for the purpose of obtaining a high exponent and a high resistance.
U.S. Pat. No. 3,928,245 discloses that instability problems occur when silica is used as a zinc oxide varistor composition additive. The latter patent improves over the former by disclosing the addition of equimolar concentrations of the oxides of barium and boron in order to enhance long term varistor stability without causing a decrease in either the varistor exponent or resistance.
It has since been determined that varistors containing the oxides of silicon, boron, and barium, contain a higher watts loss value than varistors not containing silica. It has heretofore been impossible to manufacture zinc oxide varistors for lightning arrester applications, without the addition of silica and still maintain a high exponent and high resistance. Varistors having a high watts loss value must be provided with sufficient heat sink material to ensure that they do not become overheated during a surge voltage condition and be driven into a condition known as "thermal runaway". Since varistors, in station arrester applications, are continuously subjected to a leakage current, the amount of varistor watts loss is an important design consideration.
The purpose of this invention is to provide a varistor composition having low watts loss and good long term stability without decreasing the exponent or resistance.